What's up with lousy bass on classic rock recordings?

Use a sound meter app like decibel on iPhone to measure the actual frequency response with a variety of recordings. Are you seeing response down below 40 hz or so with the recordings you know have extended bass? If not adding a proper spec’ed powered sub that is up to the task for your room and mixing it in using same sound app to measure while adjusting the sub to fill in the missing low end will make a huge difference. The key is to measure, see what’s missing and get a good sub big enough to do the job and fill in the low frequencies, not just add more of what you already have otherwise. After that if you don’t hear the bass it’s just not there to hear in the recordings you expect it to be. That’s assuming electronics used  are up to the task of delivering full range sound down to 20hz which is generally regarded as the practical low end for human ears.  Most good quality modern hifi gear should be up to the task. 

@mapman I use an SPL app on my iPhone routinely to protect my hearing and yes the low end down to 31Hz is prominent.

Did you look at my system?

I also find some older recordings to be light on the bass output. Especially true on some older LP's. I find that simply raising the frequency of the low pass filter and turning up the gain on my sub captures what's missing. 

@macg19 looks like you got the bases pretty well covered.    It does look like a larger room.  Have you ever measured using white noise?   That gives a good idea of frequency response and room acoustics. If white noise is not flat down to 20 hz with the subs in play,  then try adjusting the subs accordingly.  
 

Then you will be in a very good place to assess bass recording to recording.   Of course each will be different and range from lots of bass to virtually none.  Recordings are what they are.   If you have digital files you could always try remastering things yourself using readily available software like Audacity for example.   I use Audacity to master digital I create from my vinyl record collection. I’m sure there are even better ways.   Just a thought.   Where there’s a will there is often a way. 

@macg19 Tell me a specific classic rock track and I will give it a play and let you know what I hear and also stick the meter on it.